The main difference between single circulatory systems and double circulatory systems is that in the case of single circulatory systems, blood passes through the heart only once on each circuit around the whole of the blood circulation system of the animal.
(Conversely, in the case of double circulatory systems, blood passes through the heart twice during one complete circuit around the blood system through the body of the animal.)

Fish have single circulatory systems in which blood passes through the heart only once each time it completes a full circuit around the fish's body, including through its gills and all other organs and tissues.

Above: Diagram of a single circulatory system in a fish

More about Single Circulation Systems:

Closed blood circulatory systems include 3 aspects:

Blood
Blood is the fluid that moves through the blood system. It includes many different particles, especially

Blood Plasma - consisting mainly of water containing solutes such as electrolytes, plasma proteins and hormones

Red Blood Cells - responsible for transporting oxygen to tissues around the body

White Blood Cells - have important immune function roles e.g. in phagocytosis and production of antibodies

Blood Vessels
Blood vessels are the biological "tubes" through which blood flows. They contain the blood within the blood system and ensure that it only flows in one direction through the blood system (e.g. due to sufficient blood pressure in arteries and due to the presence of valves in the main veins of the body). In order of flow from leaving the heart to returning to the heart, the main types of blood vessels are: arteries, arterioles, capillaries, venules and veins. The main larger blood vessels (not all the individual capillaries, of which there are too many) have individual names according to their location in the body.

Heart
The heart is the pump that sends blood through the blood system at an appropriate rate for the blood to replenish the tissues as needed and to perform the functions of blood.

As applies to all closed circulatory systems, single circulation systems consist of blood, blood vessels and a heart - see more about these on the right.

The fluid (i.e. blood) contained within the network of (blood) vessels must be moved around the system in the correct direction. This is achieved by the heart continuously pumping.

In the case of single circulatory systems, e.g. as in fish, the heart receives deoxygenated blood (also described as 'oxygen-poor blood') into the atrium of the heart and pumps it through the ventricle of the heart so that it continues onwards to the gills.
Note: gills, not lungs, in the case of aquatic animals such as fish.

The deoxygenated blood is oxygenated as it passes through the gill capillaries which are tiny blood vessels whose walls are so thin that the waste product carbon dioxide can be released from the deoxygenated blood and oxygen accepted into the blood before the blood moves onwards through the blood vessels throughout the rest of the animal's body.

Also in the same way as for all closed circulatory systems, oxygenated blood supplies oxygen to the animal's cells as a result of the blood flowing through the blood vessels from larger arteries, via arterioles, into tiny capillaries where the exchange of materials that fulfil the transport functions of blood occur. (A very simple diagram of a single circulatory system in a fish is shown above.)